The quality of the output signal generated by an amplifier for amplifying a small signal is highly susceptible to common mode noise because of the high gain required to generate a useful voltage level from the relatively low voltage level of the small signal. For example, if the small signal has a maximum voltage level of 10 milliVolts, the amplifier must provide a 500 times amplification in order to generate a more useful voltage level of 5 Volts for the output signal representing the amplified small signal.
Common mode noise is often the major source of noise in an output signal generated from a small signal presented for amplification at the inputs to an amplifier, e.g., the quality of the output signal of a single-ended preamplifier is highly susceptible to common mode noise. Common mode signals can exist when a circuit contains a loop that allows current to flow from the source of the common mode signals through all signal paths of interest and return to the common mode source via any non-signal path. Common mode signals are typically created by electrical fields or gradients which are coupled into either all signal paths of interest or any other non-signal path in the circuit loop or both. These common mode signals result in common mode currents flowing through the common mode circuit loop. Any impedance misbalance between the signal leads to the common mode current will result in the common mode current generating a differential voltage between the leads with the misbalance. This results in common mode noise artifact appearing along with the signal of interest in the output of the amplifier.
In the prior art, several attempts have been made to solve the problem of eliminating or suppressing the common mode noise presented at the input to an amplifier, so that the output signal generated by the amplifier has a high small signal to common mode noise ratio, i.e., an improved common mode rejection ratio (CMRR). The prior art attempts include the following:
(1) reducing the effect of a varying external electric field by physically moving the source of the small signal away from the electric field; PA1 (2) employing a separate instrumentation amplifier to increase the CMRR by converting a single ended input to the amplifier into a differential input; PA1 (3) electrically isolating the amplifier to increase the CMRR; and PA1 (4) improving the CMRR by increasing the differential mode impedance while simultaneously decreasing the common mode impedance by adding additional circuitry which includes active components such as operational amplifiers.
In FIG. 1A, a schematic overview 10 is illustrated of a prior art electronic circuit for amplifying a small signal with a single-ended, non-isolated amplifier. An end of a common mode noise source 18 is coupled by a parasitic capacitance 15 to earth ground and another end of the noise source is coupled to a small signal source 16. The small signal source 16 is coupled through its source impedance elements (represented by a resistor 22 and a resistor 24) to an input of a single-ended amplifier 12 as well as the reference for the amplifier. The non-isolated power supply (not shown) of the amplifier is coupled to both earth and circuit ground. The impedance to the common mode current is misbalanced in the signal leads. This results in almost all of the common mode current flowing through the resistor 24 creating different voltage drops across resistors 22 and 24. The difference in the voltage drops adds to the signal source 16 and thus creates an artifact in the output signal. Since the reference for the amplifier is circuit ground, no common mode voltage is present at the input of the amplifier as in FIGS. 1B & 1C below.
FIG. 1B is a schematic overview 26 that illustrates a prior art differential amplification circuit that uses two (non-inverting and inverting) inputs to connect to the signal source 16 and make the noise source 15 common to both inputs. A signal source 16 is coupled to both inputs of amplifier 20 through its source impedance elements (represented by resistors 22 and 24). The common mode noise source 18 is coupled by the parasitic capacitance to earth ground. The noise source 18, in effect, also couples to the small signal source 16 and is present at both inputs of the amplifier. Also, the non-isolated power supply (not shown) of the amplifier is coupled to both earth and a circuit ground.
Although the prior art circuit in FIG. 1B improves the CMRR, it only provides a relatively small common mode impedance and this circuit allows the common mode voltage signal to be impressed across both inputs to the amplifier. When the common mode voltage signal is greater than the supply rails of the amplifier, the amplifier will saturate and not amplify the small signal impressed across its inputs.
In FIG. 1C, a schematic overview 30 illustrates a prior art differential amplification circuit substantially similar to the circuit shown in FIG. 1B. In this case, an isolated power supply (not shown) is used to energize the amplifier 20. A parasitic capacitance (represented by a capacitor 28) is created by the proximity of the device's ground plane to earth. Although the common mode current is lowered due to the increase in common mode impedance, the common mode signal is still present on all the inputs to the amplifier at least partly because there is both a connection to earth ground through the capacitor 28 and another connection to circuit ground. As with the circuit shown in FIG. 1B, these common mode signals may exceed the common mode input range of the amplifier leading to errors in the output. However, the circuit functions correctly at higher common mode signal levels than that shown in FIG. 1B. A disadvantage of this prior art circuit is that since both inputs are floating with respect to circuit ground, neither input can be effectively used as an external reference for such features as a serial output.
FIG. 1D illustrates a schematic overview 32 of a prior art amplification circuit substantially similar to the circuit shown in FIG. 1A. However, in this case, the amplifier 12 is energized by an isolated power supply (not shown) as in FIG. 1C. A parasitic capacitance (represented by a capacitor 28) is created by the proximity of the device's ground plane to earth and the common mode current is lowered due to the increase in common mode impedance. However, as in FIG. 1A, the impedance to the common mode current is misbalanced in the signal leads. This results in almost all of the common mode current flowing through the resistor 24 creating different voltage drops across resistors 22 and 24. The difference in the voltage drops adds to the signal source 16 and thus creates an artifact in the output signal. Also, since the reference for the amplifier is circuit ground, no common mode voltage is present at the input of the amplifier as it is in FIGS. 1B & 1C.
The present invention allows the use of the configuration shown in FIG. 1D with its attendant advantages, but removes the problem of misbalanced impedances to common mode currents in the signal leads. It does this by enclosing the electronic circuitry with two shields--an inner shield connected to circuit ground and the reference input, and an outer shield which is coupled to all other signal inputs through impedance elements. These impedance elements are typically capacitors whose value is chosen to match the capacitance formed between the inner and outer shields. This results in all signals having a balanced impedance to the common mode current which in turn results in the common mode current being balanced for all signals. Thus no conversion of common mode to differential mode takes place. In this way, the present invention enables a small signal of interest to be amplified to a usable voltage level without generating an output signal that also includes a relatively large component of common mode noise.